Susan and her brother Peter only discover Narnia after their younger siblings have already been there: their sister Lucy has been there twice but was not believed, and Edmund reaches Narnia once at the same time as Lucy, but spitefully denies it to support the suggestion that she made it up. But when the four attempt to avoid Mrs. Macready, the housekeeper of the large country house in Dorset in which they are staying during London's air raids, all four find the passage to Narnia at the back of the wardrobe they are hiding in.

In the original novel, Susan is advised to stay out of the battle, though in the 2005 movie, she saves Edmund from the White Witch's dwarf. Together with her sister Lucy, she witnesses Aslan's death and resurrection on the Stone Table. After the battle, she is crowned as Queen of Narnia by Aslan, and shares the monarchy with her brothers Peter and Edmund and her sister Lucy. She becomes known as Queen Susan the Gentle. The period of their reign is considered the Golden Age of Narnia.

Throughout the book, Susan is the voice of caution and common sense, but is often too concerned with physical comfort. At the end, after a number of years in Narnia, she counsels against pursuing the White Stag, fearing the unknown transition she and her siblings sense that the pursuit might bring. Of all the Pevensies, Susan is the most content in positions of ease and comfort, and tends to advise the others to avoid anything unpleasant.

Susan's magical horn has become an ancient relic given to Prince Caspian by his tutor, the half-dwarf magician Doctor Cornelius. When the Prince's life is threatened by King Miraz the Usurper, Caspian blows the horn and the Pevensies are magically transferred to Narnia from a railway station in England. Using the bow and arrows she has retrieved from the ruin of Cair Paravel, Susan proves her legendary prowess at archery by defeating Trumpkin the dwarf in a friendly competition. She is described as being gentle and tender-hearted, and embarrassed to defeat the dwarf. Edmund, however, later describes her as "a wet blanket". She argues against Lucy's insistence in Aslan's presence, although she later admits to having known deep down that it was true. Aslan tells Susan that she has "listened to fears", but his breath soon restores her faith, and she immerses herself in their adventures as deeply as in the first book. She later accompanies Lucy as Aslan revives the forest and river spirits of Narnia. At the conclusion of Prince Caspian, Aslan says that Susan and Peter will never enter Narnia again because they have accomplished what they needed to there.

In the 2008 film adaptation, Susan is seen in a brief scene (newly added for the film) set in front of the Strand train station in London before the children are drawn into Narnia by magic. Susan encounters a boy who recognizes her and tries to chat her up. He comments that he has seen her before and that she often sits alone. She replies that she likes being alone, and when the boy asks her name she gives him the false name "Phyllis". In Narnia, Susan and Prince Caspian are attracted to one another, but this relationship leads to a bittersweet conclusion as Susan must return to Earth. The film gives Susan a more active role during the battle scene at the end of the story. She demonstrates herself a capable combatant, slashing Telmarines with her arrows between shots, as well as clubbing them with her bow.

In The Voyage of the Dawn Treader, Susan accompanies her parents on a trip to America, while Peter is being tutored by Professor Digory Kirke. Edmund and Lucy have to stay with their relatives, the Scrubbs. Susan is considered "the pretty one of the family", which makes Lucy insecure. Lucy is tempted to recite a spell she finds in Coriakin's magic book, which will make her beautiful "beyond the lot of mortals", and she pictures a plain-looking Susan jealous of her beauty - clearly hinting at a sibling rivalry not evident before. Susan is not very good at school work and acts old for her age (14 at the time).

The 2010 film adaptation shows Susan in a few brief scenes newly added for the film. She is seen at the beginning of the film writing a letter to Edmund and Lucy. Later in the film, Lucy steals the page with the beauty spell from Coriakin's book and at night recites the spell. Looking in the cabin's full-length mirror, she sees herself, grown-up and beautiful, in effect becoming Susan. The mirror swings aside to reveal a society lawn party back in Lucy's world, and she steps through into the party to be joined by both Peter and Edmund, both of whom address her as "Susan". When she tries to remind them about Narnia, to her growing horror, neither of them seem to know what she's talking about or to know anything about a younger sister named Lucy. Lucy then wakes up in the ship's cabin and is confronted by Aslan about what she has done. Aslan tells her not to judge herself by the standards of others, and reminds her that it was only through her actions that her siblings became aware of Narnia in the first place.

In The Horse and His Boy, set during the Pevensie siblings' reign in Narnia, Susan plays a minor part. She is described as a gentle lady with black hair falling to her feet. Shasta thinks her the most beautiful woman he has ever seen. Susan has become a motherly or sisterly figure to Prince Corin of Archenland following the death of his mother. She is asked to make a diplomatic marriage to the Calormene Prince Rabadash. She finds him gallant at tournaments in Narnia, but tyrannical and repellant on his home ground. The prince intends to hold her in Tashbaan by force; but her guarded rejection and subsequent escape from Calormen on The Splendor Hyaline with her brother Edmund and their entourage lead the Prince to rash action. He seeks the secret approval of his father, the Tisroc, for his plan to attack Archenland as a means of capturing Susan, and in the hope of subsequently conquering Narnia.

In The Last Battle, Susan is conspicuous by her absence. Peter says that she is "no longer a friend of Narnia", and (in Jill Pole's words) "she's interested in nothing now-a-days except nylons and lipstick and invitations." Similarly, Eustace Scrubb quotes her as saying, "What wonderful memories you have! Fancy you still thinking about all those funny games we used to play when we were children," and Polly Plummer adds, "She wasted all her school time wanting to be the age she is now, and she'll waste all the rest of her life trying to stay that age. Her whole idea is to race on to the silliest time of one's life as quick as she can and then stop there as long as she can." Thus, Susan does not enter the real Narnia with the others at the end of the series. It is left ambiguous whether Susan's absence is permanent, especially since Lewis stated elsewhere that:

The books don't tell us what happened to Susan. She is left alive in this world at the end, having by then turned into a rather silly, conceited young woman. But there's plenty of time for her to mend and perhaps she will get to Aslan's country in the end... in her own way.[4]

In his Companion to Narnia, Paul F. Ford writes at the end of the entry for Susan Pevensie that "Susan's is one of the most important Unfinished Tales of The Chronicles of Narnia", but adds in Footnote 1 for that entry:

This is not to say, as some critics have maintained, that she is lost forever ... It is a mistake to think that Susan was killed in the railway accident at the end of The Last Battle and that she has forever fallen from grace. It is to be assumed, rather, that as a woman of twenty-one who has just lost her entire family in a terrible crash, she will have much to work through; in the process, she might change to become truly the gentle person she has the potential for being.

Fantasy author Neil Gaiman's 2004 short story "The Problem of Susan" depicts its protagonist, Professor Hastings (who strongly resembles an adult version of Susan), dealing with the grief and trauma of her entire family's death in a train crash, as she is interviewed by a college literature student regarding her opinion on Susan's place in the Narnia books.[5]Harry Potter author J. K. Rowling has also commented on the same issue:[6]

“

There comes a point where Susan, who was the older girl, is lost to Narnia because she becomes interested in lipstick. She's become irreligious basically because she found sex. I have a big problem with that.

^Bartels, Gretchen, "Of Men and Mice: C. S. Lewis on Male–Female Interaction", Literature and Theology, 22 (3): 324–338, doi:10.1093/litthe/frn026, This interpretation has become known amongst Lewis scholars and enthusiasts as the Problem of Susan, which was first coined by the writer Neil Gaiman in his short story by the same name.

^Abate, Michelle Ann; Weldy, Lance, eds. (2012), C.S. Lewis: The Chronicles of Narnia, Palgrave Macmillan, p. 4, Both [J. K.] Rowling and [Neil] Gaiman are concerned with the manner in which Susan Pevensie’s character becomes represented ... [Gaiman's story] has become eponymous with the issue"